1. Field of the Invention
The invention relates to a control apparatus and a control method for an automatic transmission, and more particularly to direct shift control in which two or more frictional engagement elements are engaged while disengaging two or more frictional engagement elements.
2. Description of the Related Art
Constitutional components of an automatic transmission for a vehicle include a planetary gear constituted by rotary elements such as a sun gear, a ring gear and a carrier, and frictional engagement elements. In this type of automatic transmission, a plurality of gear positions are achieved by switching a torque transmission path from an engine in accordance with a combination of engaged frictional engagement elements. In particular, an automatic transmission having a large number of gear positions (a sixth speed automatic transmission, for example) is provided with both a plurality of clutch elements serving as input elements and a plurality of brake elements serving as reaction elements.
In this type of automatic transmission, when a shift is performed from a current gear position to a gear position that is higher or lower than the current gear position by a plurality of positions in accordance with a shift operation, an acceleration request, or the like, the shift may be performed by switching both the input element and the reaction element. In this case, the shift is sometimes performed by switching either the input element or the reaction element to achieve an intermediate gear position, and then switching the other.
Shift control for switching the gear position in two stages via an intermediate gear position is conventionally available. For example, a shift control apparatus disclosed in International Patent Application Pamphlet WO2003/029699 includes means for determining a dual changeover shift from an Nth speed achieved by at least engaging a first friction element and a second friction element to an N-αth speed achieved by at least disengaging the first friction element and the second friction element and engaging a third friction element and a fourth friction element. An intermediate gear position achieved by engaging the second friction element and the third friction element exists between the Nth speed and the N-αth speed. This shift control apparatus also includes: means for achieving the shift from the Nth speed to the N-αth speed when a dual changeover shift is determined by at least disengaging the first friction element and engaging the fourth friction element, reducing an engagement force of the second friction element before a gear ratio reaches a gear ratio corresponding to the intermediate gear position, and at least disengaging the second friction element and engaging the third friction element after the gear ratio passes the gear ratio corresponding to the intermediate gear position; means for deciding to prohibit modification of a target gear position from the N-αth speed until shift control is complete at a predetermined timing following the start of the shift from the Nth speed to the N-αth speed; and means for reconfirming an intention of a driver to perform a shift when the gear ratio reaches the gear ratio corresponding to the intermediate gear position and permitting modification of the target gear position to a gear position corresponding to the intention of the driver when the target gear position differs from the N-αth speed, even if modification of the target gear position is prohibited.
According to the shift control apparatus for an automatic transmission disclosed in International Patent Application Pamphlet WO2003/029699, when the driver intends to modify the gear position, a shift can be achieved in a short period of time, and when the driver does not intend to modify the gear position, an oil pressure of a high clutch is reduced before reaching a gear ratio corresponding to a fifth speed. Hence, the shift is not determined, and therefore a plurality of shift shocks do not occur during a dual changeover. Accordingly, when the driver does not intend to modify the gear position, a smooth dual changeover shift can be achieved.
Although International Patent Application Pamphlet WO2003/029699 describes shift control for switching the gear position in two stages via an intermediate gear position during a shift from a current gear position (Nth speed) to a gear position (N-αth speed) been higher or lower than the current gear position by a plurality of positions, no mention is made of shift control (direct shift control) for switching the gear position in a single stage without passing through an intermediate gear position. Accordingly, measures to be taken when the target gear position changes during direct shift control that does not pass through an intermediate gear position are not disclosed.